In electronic devices, such as personal digital assistants (PDAs), laptop computers, office automation equipment, medical equipment, and car navigation systems, touchscreens are widely used as their display screens that also serve as input means.
There are a variety of touchscreens that utilize different position detection technologies, such as optical, ultrasonic, capacitive, and resistive technologies. A resistive touchscreen has a configuration in which an optically transparent conductive material and a glass plate with an optically transparent conductive layer are separated by spacers and face each other. A current is applied to the optically transparent conductive material and the voltage of the glass plate with an optically transparent conductive layer is measured. In contrast, a capacitive touchscreen has a basic configuration in which an optically transparent support has a transparent conductor layer thereon and there are no movable parts. Capacitive touchscreens, which have high durability and high transmission, are applied, for example, to in-car equipment.
As optically transparent electrodes (optically transparent conductive material) for touchscreens, optically transparent supports having an optically transparent conductive film made of ITO (indium tin oxide) formed thereon have been commonly used. However, there have been problems of low total light transmittance due to high refractive index and high surface light reflectivity of ITO conductive films. Another problem is that ITO conductive films have low flexibility and thus are prone to crack when bent, resulting in an increased electric resistance. As an optically transparent conductive material having an optically transparent conductive film which is an alternative to the ITO conductive film, an optically transparent conductive material having a high light transmittance and a high conductivity, the material being prepared by forming, for example, a mesh-like pattern of metal thin lines on a support and by forming, for example, a mesh-like pattern of metal thin lines with a specific line width and/or pitch of the metal thin lines, a specific pattern shape, etc. on a support, is disclosed in, for example, Patent Literature 1.
As a method for forming a microscopic metal pattern, a semi-additive method for forming a metal pattern, the method comprising making a thin catalyst layer on a base material, making a resist pattern on the catalyst layer, making a laminated metal layer in an opening of the resist by plating, and finally removing the resist layer and the base metal protected by the resist layer, is disclosed in, for example, Patent Literature 2 and Patent Literature 3.
Also, in recent years, as a method for forming a metal pattern, a method using a silver halide photosensitive material as a precursor to a conductive material has been proposed. For example, Patent Literature 4, Patent Literature 5, and Patent Literature 6 disclose a technology for forming a metal silver pattern by a reaction of a conductive material precursor having a physical development nucleus layer and a silver halide emulsion layer in this order on an optically transparent support with a soluble silver halide forming agent and a reducing agent in an alkaline fluid. The patterning by this method can reproduce uniform line width. In addition, due to the highest conductivity of silver among all metals, a thinner line with a higher conductivity can be achieved as compared with other methods, and thus an optically transparent conductive film having a high total light transmittance and a reduced electric resistance can be obtained. An additional advantage is that an optically transparent conductive film obtained by this method has a higher flexibility, i.e. a longer flexing life as compared with an ITO conductive film.
In the cases of a resistive touchscreen and a capacitive touchscreen, the optically transparent electrode has an optically transparent electrode unit, which is placed on a display and is used for manual operation, and a peripheral electrode unit, which is disposed outside the display and is used for transmitting the electric signals detected in the optically transparent electrode unit to the outside. In the cases where an ITO conductive film is used in the optically transparent electrode unit, the production is generally not very efficient because, besides the step of forming the ITO conductive film, an additional step of, for example, forming a peripheral electrode unit using a silver paste or the like on top of the ITO conductive film is required. In contrast, the above-mentioned method using a silver halide photosensitive material as a conductive material precursor is a very efficient method by which, as described in Patent Literature 7, for example, a grid-like optically transparent electrode unit formed of a silver pattern and a peripheral electrode unit can be simultaneously produced.
However, despite being a noble metal, silver is not a stable metal and easily reacts with sulfur in the air to make silver sulfide, for example. Therefore, when the above-mentioned method using a silver halide photosensitive material as a conductive material precursor is employed, there is a problem regarding the stability of silver. Furthermore, in the cases where an electrode pattern is formed using silver, an inexplicable phenomenon is observed. That is, the stability of the electrode pattern varies depending on the shape of the pattern. The problem is that, although most of the pattern is hardly corroded, only a certain part, in particular, a grid-like optically transparent electrode unit electrically connected with a long peripheral wire unit is extremely susceptible to corrosion. In the method using a silver halide photosensitive material as a conductive material precursor, for further improved conductivity, a method, for example, in which the prepared grid-like optically transparent electrode unit formed of a silver pattern is plated with another metal by electroless plating is also employed. However, depending on the shape of the pattern, the electroless plating may result in non-uniform plating or the like although the reason is also unknown. In some cases, there exists a part where no plating result can be observed.
Meanwhile, Patent Literature 8 proposes changing the width of the peripheral wiring, and in this case, the peripheral wire is made of a metal, for example, molybdenum/niobium, and the optically transparent electrode unit is formed of an ITO conductive film.